Max Karl Ernst Ludwig Planck (23 April 1858–4 October 1947) was a German theoretical physicist whose research on blackbody radiation introduced the quantum of action, now called the Planck constant. His radiation theory of 1900 became a foundational contribution to quantum mechanics, although he initially resisted some of its implications for classical physics. He received the Nobel Prize in Physics for 1918 for discovering energy quanta; the reserved award was announced in 1919. (mpg.de)
Early life and academic career
Planck was born in Kiel and moved with his family to Munich in 1867. He studied at the universities of Munich and Berlin between 1874 and 1879, with Hermann von Helmholtz and Gustav Kirchhoff among his teachers. His doctoral dissertation, completed in Munich in 1879, examined the second law of thermodynamics. After his habilitation in 1880, he taught as a private lecturer in Munich. He became an associate professor of theoretical physics at Kiel in 1885 and moved to Berlin in 1889, becoming a full professor in 1892. He retired from his university chair in 1926. (mpg.de)
His early research centered on thermal processes, irreversibility, and entropy. These interests shaped his later investigation of radiation: he sought a universal physical explanation of how matter and radiation reach thermal equilibrium. Beginning in 1897, he approached this problem through electrodynamics, initially hoping to derive the radiation spectrum without abandoning established physical principles. (mpg.de)
The radiation problem
A black body is an idealized object that absorbs all incident electromagnetic radiation. At thermal equilibrium, its emitted spectrum depends on temperature rather than on its material composition. An approximately black cavity with a small opening provides an experimental realization. Measurements made in Berlin during the 1890s and around 1900 supplied increasingly precise evidence against which proposed radiation laws could be tested. The problem connected the study of light with the laws of heat. (mpg.de)
Planck initially supported Wilhelm Wien’s radiation formula, but new measurements showed that it failed in part of the spectrum. On 19 October 1900, he presented a replacement formula that agreed with the available observations. The empirical success of this law preceded a satisfactory explanation of why it worked. Planck therefore turned to the statistical counting approach associated with Ludwig Boltzmann, despite his earlier reservations about that method. (mpg.de)
Energy quanta
On 14 December 1900, Planck presented a derivation of his radiation law using a quantum hypothesis. To count the possible distributions of energy among the resonators in his model, he divided the total energy into finite elements of magnitude
[ \varepsilon=h\nu, ]
where (\nu) is frequency and (h) is the new universal constant. Unlike an arbitrarily small mathematical subdivision, this energy element could not be allowed to approach zero without losing the successful radiation formula. This became a crucial step toward energy quantization. (max-planck.mpg.de)
The derivation connected entropy with probability through the counting of possible microscopic arrangements, drawing on ideas central to statistical mechanics. In his Nobel lecture, Planck explained how the search for a radiation law had led him to the quantum of action. His original work concerned the statistical treatment of resonators and their energy; it should not simply be equated with the later claim that freely propagating light consists of particles. (nobelprize.org)
Albert Einstein extended the quantum idea to light itself in 1905, using it to explain the photoelectric effect. This light-quantum concept developed into the modern understanding of the photon. Planck’s hypothesis and Einstein’s interpretation were related but distinct contributions. Planck remained skeptical for years about the need for a complete reconstruction of classical physics. (nobelprize.org)
Recognition and scientific institutions
Planck became a member of the Prussian Academy of Sciences in 1894 and served as permanent secretary of its physical-mathematical class from 1912 to 1938. He was rector of Berlin University in 1913–1914. His Nobel lecture, delivered on 2 June 1920, described both the origins of quantum theory and the unresolved questions surrounding it. In 1929, the German Physical Society established the Max Planck Medal, awarding its first medals to Planck and Einstein. (max-planck.mpg.de)
From 1930 to 1937, he was president of the Kaiser Wilhelm Society, a major German research organization. Under Nazi rule, he sought to preserve its institutions and intervened on behalf of some threatened colleagues. He also arranged a memorial meeting for Fritz Haber in 1935. These actions did not prevent the organization’s subordination to the regime. (mpg.de)
Family, final years, and namesakes
Planck married Marie Merck in 1887; after her death in 1909, he married Marga von Hoesslin in 1911. Several of his children died before him. During World War II, an air raid destroyed his Berlin home in February 1944, including scientific notebooks and diaries. His son Erwin, arrested following the failed July 1944 attempt against Hitler, was executed on 23 January 1945. Planck moved to Göttingen in 1945 and died there on 4 October 1947. (mpg.de)
The Max Planck Society adopted his name during the postwar reorganization of German research. A British-zone organization was established in 1946; the present society was founded in Göttingen on 26 February 1948, after his death. His name also remains attached to the constant (h), whose exact value in the International System of Units is (6.62607015\times10^{-34}) joule-seconds and which underpins the definition of the kilogram. (mpg.de)